MIPS: Allow direct allocation in old pointer space.

Port r13940 (a39765e7)

BUG=

Review URL: https://codereview.chromium.org/12413026

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13979 ce2b1a6d-e550-0410-aec6-3dcde31c8c00